Need Help With Experiment

Brent Gilbert bg005d at uhura.cc.rochester.edu
Tue Apr 23 16:57:33 EST 1996

In <Pine.SUN.3.93.960423100223.4925B-100000 at eskimo.com> KM <mirza at eskimo.com> writes:

>On Wed, 17 Apr 1996, David R. Boone wrote:

>> Terry J. Peek wrote:
>> > I am trying to differentiate between Serratia marcescens and a gram pos. cocci
>> > that I suspect is Staph epidermidis or S. aureus. I've placed the culture on
>> > 10% NaCl agar. I'll check the results later this morning. What is the best way
>> > to seperate these organisms using the resources of a small college lab? Is my
>> > first step in the right direction? Any suggestions would be appreciated.
>> > Thanks.       Terry
>> Terry,
>> There are lots of differences between Serratia marcescens and the two cocci you 
>> mentioned.  If you know an organism is one of these three species, it is 
>> relatively easy to distinguish Serratia marcescens.  Many strains of Serratia 
>> marcescens when grown as surface colonies produce a red pigment (in contrast, 
>> Staphylococcus epidermidis is usually light colored and Staphylococcus aureus is 
>> usually dark yellow or golden (the species epithet refers to gold).  The color of 
>> colonies is really not very taxonomically significant (in fact, <90% of Serratia 
>> marcescens strains produce the red pigment), but if you want a quick and dirty 
>> discriminator, this may be it.  From a taxonomic standpoint, probably the most 
>> reliable differentiating characteristics are morphological: Serratia are 
>> gram-negative rods, whereas staphylococci are gram-positive cocci, often arranged 
>> in grape-like clusters.
>> -- 
>> David R. Boone
>> Professor of Environmental Microbiology
>> Oregon Graduate Institute, Portland
>> 503-690-1146
>> boone at ese.ogi.edu
>> http://www.ese.ogi.edu/ese_docs/boone.html
>I am puzzled at the 90% of S.marcescens and production of pigment.
>Prof.Boone where is that reference, my understanding of pig production was
>a lot smaller than 90%.

>May be in the envoirnment isolates are differeent, I am a clinical  man,
>my observation of clinical isolates is a lot different.
>Hope to hear ur reply.

The most selective way to discriminate would be to plate on Maconkey agar.
The crystal violet is selective for gram-negative organisms. If it is indeed
S. marscenses, the culture will grow. If they are Gram-positive, no growth
will be seen on the plates. Hektoen-enteric plates serve the same function.

Brent Gilbert (bg005d at uhura.cc.rochester.edu)

"If you choose not to decide, you still have made a choice" -Neil Peart

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